Method of preparing oxidation resistant materials

ABSTRACT

Preparation of iron-base alloys, particularly in the form of regenerator cores and other similar matrices, by the codiffusion of aluminum and chromium, using aluminum-iron alloy powder and chromium, aluminum-iron alloy powder and chromium-iron powder or Al-Cr alloy powder as sources of the aluminum and chromium and an atmosphere of mixed H2 and HF to accomplish in situ formation of the aluminum and chromium and their diffusion, and alloying with the iron-base alloy. Assemblies may be bonded to form an integrated structure along with the heating for coduffusion of aluminum and chromium.

United States Patent [191 Davis et al.

[ METHOD OF PREPARING OXIDATION RESISTANT MATERIALS [75] Inventors: Royal E. Davis, Farmington;

Amedee Roy; Claude Belleau, both of Troy; Gordon E. Allardyce, Dearborn Heights, all of Mich.

[73] Assignee: Chrysler Corporation, Highland Park, Mich.

22 Filed: Dec. 27, 1972 211 App]. No.: 318,785

[52] US. Cl 29/460, 29/494, 29/527.2, 117/22, 117/107.2 P, 117/131 [51] Int. Cl B23p 3/00, B23p 19/04 [58] Field of Search 29/494, 527.2, 460; 117/131, 22,1072 P [56] References Cited UNITED STATES PATENTS 2,801,187 7/1957 Golmiche 29/494 3,061,462 10/1962 Samuel. ll7/l3l UX 3,096,205 7/1963 De Grusto 117/131 X 3,257,227 6/1966 Seelig 117/l07.2 P X 3,340,054 9/1967 Ward et a1. 1 17/22 UX 3,365,327 1/1968 Pryear et a1. 117/107.2 P

[111 3,807,030 1 Apr. 30, 1974 Primary Examiner-.1. Spencer Overholser Assistant ExaminerRonald .1. Shore [5 7] ABSTRACT Preparation of iron-base alloys, particularly in the form of regenerator cores and other similar matrices, by the codiffusion of aluminum and chromium, using aluminum-iron alloy powder and chromium, aluminum-iron alloy powder and chromium-iron powder or Al-Cr alloy powder as sources of the aluminum and chromium and an atmosphere of mixed H and HF to accomplish in situ formation of the aluminum and chromium and their diffusion, and alloying with the iron-base alloy. Assemblies may be bonded to form 'an integrated structure along with the heating for coduffusion of aluminum and chromium.

17 Claims, 15 Drawing Figures PATENTEDAPR30 19M (1807030 SHEET 1 [IF 4 PMENYEDWRWW 3807.030

SHEET E OF 4 7 ma WM 4' METHOD or PREPARING OXIDATION RESISTANT MATERIALS BACKGROUND This invention relates generally to materials and to matrix structures of oxidation resistant iron-base alloys. The term iron-base alloy is used herein to define low carbon mild steel and similar iron-base alloys. This invention relates to a method of diffusing aluminum and chromium into iron-base alloys and iron-base matrix assemblies and the simultaneous bonding of iron-base alloy assemblies to form integral structures. The invention is specifically directed to regenerator cores for turbine engines although it is applicable to similar matrix structures wherein low carbon, mild steel and iron parts form various passageways, the walls of which are to be diffusion alloyed with chromium and aluminum and the parts of which are to be bonded together. The term mild steel or low carbon steel is commonly used and is used herein to describe well-known steels, particularly commercial steels, containing less than about 0.25 percent by weight carbon, balance iron and the usual impurities. Examples of some commercial low carbon irons are Armco Supersoft (Armco Steel Co.), Bethnamel (Bethlehem Steel Corp.) and Vitrenamel (United States Steel Corp.). An example of a low carvapor phase diffusion process is unacceptable because of the high pressure drop across such honeycomb type matrix structures. Metallic vapors are found to deposit preferentially on the entering surfaces resulting in eventual plugging of the passage and poor distribution of the metals carried by the vapor.

Other difficulties in accomplishing diffusion exist due to the nature of the specific materials used, i.e., the aluminum and chromium. For example, providing oxidation resistant iron-base alloys by chromium diffusion requires high chromium levels which ordinarily results in sigma formation and in the formation of other brittle Fe-Cr compounds when the material is used in high temperature environments as are turbine engine regenerators. On the other hand, aluminum tends to form brittle alloys when diffused alone into iron.

Attempting to simultaneously diffuse metallic aluminum and metallic chromium has been unsatisfactory heretofore also. Chromium requires high-temperatures in excess of about 1,200" F. to initiate diffusion. At such a temperature, metallic aluminum wets the workpiece surface and prevents the difi'usion of the chromium into it.

In addressing itself to these problems, the present invention uses source materials for the aluminum and chromium which in combination with a certain atmosphere form proper amounts of aluminum and chromium in situ for codiffusion thereby overcoming many of the problems typically associated with the diffusion of these elements.

SUMMARY OF THE INVENTION 5 This invention makes use of a novel approach in order to codiffuse aluminum and chromium and thereby provide oxidation resistant material. In such an approach, the source of the diffusing metals (aluminum and chromium) is placed in close proximity to the substrate. A slurry technique has been found to be very successful in this invention as a means of distributing the source of materials, directly on a substrate, such as the surfaces of a matrix assembly in the form of a regenerator core and in producing good alloying and bonding of the parts thereof by the diffusion of the aluminum and chromium as provided herein. Both chromium and aluminum are formed in situ at the substrate and diffused into the substrate material during a heating cycle in the presence of hydrogen and HF gas. The source of the aluminum is an iron-aluminum or chromium-alumin'um alloy while the source of chromium may be chromium per se, an aluminum-chromium alloy or an iron-chromium alloy. Hereinafter when reference is made to chromium or Cr" as a source material it should be taken to includenot only the metal per se but Cr-Fe and Cr Al alloys as well as mixtures of Cr and Cr-Fe or Cr-Al alloys. I

It is a general object of this invention to diffuse aluminum and chromium into iron-base alloys, such as low carbon mild steel and low carbon iron materials, by a new and improved method wherein the actual diffusion elements, aluminum and chromium, are formed in'situ.

It is also an object to thereby provide oxidation resistant materials and matrix structures from relatively low cost materials, namely, low carbon iron and mild steels, by such codiffusion. I

It is a specific object to provide low cost oxidation resistant regenerator cores for turbine engines.

It is an object to provide a method wherein processing techniques of reasonable cost may be used for making oxidation resistant materials and matrix structures from low carbon iron or mild steel.

,It is also an object to simultaneously bond a matrix assembly into an integral structure during diffusion.

It is still another object to use iron-aluminum or chromium aluminum alloys as a sourceof aluminum and chromium, chromium aluminum alloys or ironchromium alloys as source materials of chromium for diffusion into low carbon iron or mild steel to provide oxidation resistant materials and structures.

It is. another object to provide useful oxidation resistant materials from low carbon iron or mild steel.

It is also an object to provide regenerator cores of a novel relatively inexpensive material.

These and other objects of the invention will become apparent from the following description and drawings.

BRIEF DESCRIPTION OFTHE DRAWINGS FIG. 4 is a graph illustrating the effect of atmosphere flow rate on themethod of the invention;

FIG. 5 is a graph illustrating the effect of temperature and time on the method of the invention;

FIG. 6 is a graph illustrating the effect of slurry composition of the method of the invention with Fe-Al +Cr as the source material;

FIG. 7 is a graph illustrating the effects of the diffusion atmosphere composition on the method of the invention with Fe-Al Cr as the source material;

FIG. 8 illustrates and classifies the oxidation resistance of various Al-Cr materials at 1,400 F. in circulating air,-the results being expressed in terms of weight gain due to oxidation;

FIG. 9 is a graph illustrating the oxidation resistance of various portions of a specific regenerator matrix sample, 93.5 percent Recovery meaning that 93.5 percent of the slurry materials diffused and alloyed.

FIG. 10 is a graph illustrating variations in slurry retention during dipping in terms of withdrawal rate, SWG meaning slurry weight gain as a result of dip- P FIGS. 11 and 12 are graphs illustrating the variations in slurry retention with changes in viscosity for several binder compositions, P & S meaning Pierce and Stevens Co.

FIGS. 13 and 14 illustrate slurry distribution through the cross-section of a core sample resulting from dipping and its effect on resultant composition therethrough.

FIG. 15 is a graph illustrating oxidation resistance of the cold and hot faces of a regenerator core sample according to the invention having compositional variation, the alloy distribution curves being plotted on the lower ordinate, the centered curve being an oxidation weight gain curve plotted on the upper ordinate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While this invention is applicable to oxidation resistant materials comprised basically of iron-base alloys and to the preparation of such materials, particularly the fabrication of matrix structures of such materials, it will be described in connection with one type of such matrix structure, the preferred embodiment herein, to which it is particularly applicable i.e., turbine engine regenerator cores of the type shown in FIGS. 1, 2 and 3. Such a core, when complete, consists of a rim 10, a hub 12 and a matrix portion 14, which is best shown in detail in FIG. 3. As can be seen from FIGS. 1 and 2, the regenerator is a relatively flat, round structure with a plurality of passageways extending through the matrix for the flow of gases. The passageways in the particular design shown are formed by alternately positioned corrugated layers of low carbon iron stock and flat layers of low carbon iron stock. Other variations and designs are known. To form an integral structure these parts are bonded together and lastly, the rim and hub are attached. The method of the invention is preferably performed on the matrix of the core prior to the attachment of the rim and hub.

materials are aluminum-iron and either chromium or chromium-iron or the source materials are aluminumchromium alloys, which are placed in contact with the workpiece and heated at high temperatures in a certain reducing atmosphere, herein also termed a diffusion. These source materials are believed converted in the atmosphere to fluoride salts and then reduced to the metallic stateat the surface of the iron workpiece. The diffusion atmosphere consists essentially of hydrogen (H and hydrogen fluoride (HF) gases which together promote the requisite fluoridesalt formation and subsequent in situ reduction to the respective metallic constituents, i.e., aluminum and chromium.

Duringthe high temperatureheating in the diffusion atmosphere, the following reactions, among others, are believed to take place more or less concurrently.

The first two reactions promote the formation'of fluorides. The other reactions indicate the reduction 'of these fluorides and intermediate fluorides by either hydrogen or the metals. Furthermore, the last three reactions show the actual deposition of chromium and aluminum on the surface of the workpiece. The regeneration of hydrogen and hydrogen fluoride and the deposition of chromium and aluminum on the workpiece surface favor completion of these reactions. With the use of a static atmosphere during the heating cycle, the volatile constituents are better retained within the passages of a matrix, such as the preferred embodiment, to allow reaction with the low carbon mild steel stock. In this connection-reference is made to FIG. 4 which demonstrates that the amount of materials diffused increases as flow rate decreases and is best when the atmosphere is static. The amount of diffused material is expressed in terms of the approximate percent reacted and may include minor amounts trapped in some passages but not actually reacted. This is also referred to as the percent recovered.

Pure aluminum is an active reducing agent. If used in its elemental form, it will result timewise in the premature reduction of the chromium fluorides to metallic chromium and monoaluminum fluoride at too low a temperature for the effective diffusion of aluminum into the iron. For this reason, among others, this invention substitutes aluminum alloys for pure aluminum as a source material. The preferred iron-aluminum alloy, preferably 1:1, for example, is much less reactive and has a much higher melting point than aluminum alone. With iron-aluminum, premature reaction at low temperature is delayed until a more favorable temperature is reached and consequently a higher aluminum and chromium alloy content is produced during heating and diffusion according to the method of this invention.

It has been determined by testing that the preferred method for contacting the source materials and the workpiece comprises dipping the workpiece into a slurry containing the suspended source materials. The following procedure is typical in the preparation of matrix assemblies according to this invention.

Procedure I. Decarburizing In the case of materials and assemblies using mild steel, carbon removal is usually necessary. This may be accomplished by placing the material or assembly into a suitable heat resistant container. Exposure to wet II for about l- /z hour at about 1,600 F. usually accomplishes decarburization. In the case of an assembly, an additional advantage is provided due to the bonding (referred to hereinbelow as a prebonding simple diffusion heat treatment) which occurs under these conditions. When iron is used this step is not necessary.

2. A preferred-slurry consists, for example, of ironaluminum (I:l alloy composition, by weight) powder and chromium powder mixed in the 4:5 ratio by weight and suspended in a binder such as Pierce & Stevens Binder No. 9658, which is a solution of an acrylic resin in toluene. Additions of aluminum palmitate may be used to control the viscosity.

3. The matrix assembly is put together and clamped,

brazed, decarbu'rized, or the like, for temporarily holding it together. It.may be cleaned and then coated with the above slurry, preferably by dipping and preferably to obtain a weight gain of' about 30 percent. Regenerator core samples made from 0.002 inch stock exhibiting a weight gain of about 5 mg/cm of surface area were found to be acceptable for turbine engine use.

The assembly is next sealed in a suitable container which is placed in a furnace and heated up to about 700-800 F. under a flow of argon substantially to remove the binder vehicle. I 5. Thereafter, a diffusion atmosphere of hydrogen and hydrogen fluoride (about 1 percent hydrogen fluoride by volume, balance hydrogen, is preferred although about l-5 percent is acceptable) is introduced into the container. This can be achieved by long time purging or by evacuating the container and refilling .with the H -HF atmosphere.

6. A positive pressure (about 4 to 6 inch of oil on an oil manometer) is preferably maintained in the container during the heating and diffusing-bonding cycle, which is, for example, preferably about two hours at a holding temperature of about 2,000 F. for stock having a thickness on the order of 0.002 inches.

7. After cooling to below about 1,000 F., the container is purged with argon until room temperature is reached.

8. The assembly may be weighed upon removal from the container, after loose residue has been blown out, to determine the amount of source materials used. This was the basis for the data in the graphs of FIGS. 4, 6 and 7 for percent reacted or recovered.

9. Fluoride residues may subsequently be eliminated by heating the assembly for approximately one hour at temperatures above l,700 F. in a wet hydrogen atmosphere.

Source Materials Cr- Al alloy of 30 percent'Cr, balance Al has been used. Commercially available alloys such as 15Cr- Al, 20Cr-80Al and 66Cr-34Al may be used also. If the alloy is to be prepared as a powder for use in a slurry the l5-60 percent Cr, balance A] should be used because it is brittle and easily powdered. Fe-Cr alloy of 67.2 percent Cr balance Fe, a commercial alloy has been used. Low carbon, low silicon ferrochromes are desirable in which the carbon is less than about 0.I0 percent, the silicon is less than 2 percent and the chromium runs about 64-75%.

Fe-Al alloy of 50 percent Fe 50 percent Al has been used. It is a commercially available material. Use of an alloy of about 45 70 percent Al provides one which is rather brittle and easily powdered for use in a slurry. The 50-50 alloy typically sold for use in permanent magnets is satisfactory.

Evaluation of Samples Oxidation resistance at l,400 F. is the principal quality criterion of workpieces treated by the method of this invention. Samples were tested at that temperature in a circulating air furnace. Their weight gain, in milligrams per square centimeter, was recorded for 2, 24, 48, I00, 500, 1,000 and 2,000 hours. They were also examined metallographically to determine their condition and mode of failure if any. Cycling from room temperature to about l,400 F. was obtained by withdrawing all the samples from the oxidation testing furnace when some of them were to be weighed. Inthe case of uniform oxidation, a weight gain rate of 0.5 rng/cm hours appeared to be an acceptable maximum limit. v a

In the case of regenerator core samples, bonding of the corrugated and flat stock forming the regenerator core assemblies was evaluated qualitatively, under a low power microscope, by mechanical prodding at the joints with a suitable tool, such as a pick. Metallographic examination was also used to determine the depth of diffusion, detect any anomaly of the microstructure and confirm the quality of the bonds. The actual chromium and aluminum content of representative samples was determined bothbywet chemical analysis and X-ray fluorescence analysis.

Results therefore, include residual source materials, which do not contribute to the oxidation resistance of the substrate. However, these figures, as the weight percent of slurry constituents retained in the regenerator after the diffusion cycle, were found to be a good indication of the efficiency of the process. When percent reacted or recovered" is over 90 percent, a slurry weight gain of 23 percent has been found to yield satisfactory oxidation results. Because of the sintering of residual powsults in a physical growth of 0.75 to 1.0 percent. in the higher contact pressure of the stock at processing temperature and favors more effective bonding.

der, analysis results are only a measure of total chro- 5 The samples referred to herein were segments cut mium and aluminum and not of the effective amount of from 10 inch diameter cores previously diffusion alloying elements. bonded (without Al-Cr diffusion) to prevent the vari- Since the strength of diffusion bonds is a function of ous layers of flat and corrugated stock from coming many variables including the pressure at the points of apart during sectioning and subsequent processing. contact, evaluation of bonding o'n regenerator core 10 Samples were approximately 1 inch by 1 inch in section samples, when segments of wound cores are used, is and the length was the full thickness of an actual regenonly partially representative of the bonding of a fullerator core, i.e., 3.5 inches. size unit. In wound cores, the flat and corrugated strips, The terms percent A1 Top and Bottom and percent forming the matrix assembly, tend to maintain a uni- Cr Top and Bottom refer to the actual Cr and A1 conform contact pressure between the successive layers. In 15 tents by analysis of the top or bottom one-half inch of addition, diffusion of chromium and aluminum into the the samples. Top and bottom refer to the position of core matrix sufficient to make it oxidation resistant rethe sample when dipped in the slurry.

TABLEI 100 hrs. oxid. at 1400F. Percent Percent Percent Percent (wt. gain mglcm Sample Source Percent H (l 7 Cr l H Al A1 I I I No. Conditions materials SWG* Recovery** (top) (bottom) (top) (bottom) Top Bottom 9721......... 2 hrs/2000 static FeA1+ Cr 26.8 89.8 6.4 5.3 0.107 0.108 1% HF, balance (3:2 by H weight). 9722 2 hrs/2000 F. static FeA1+ Cr 27.2 94.0 7.8 6.1 .085 .086

1% HF,ba1ance (1:1 by H weight). 9921 2 hrs/2000F. static FeAl Cr 30.4 94.2 8.6 11.0 2.8 2.8 .175 .098

l%-HF, balance (4:5 by 1 H weight). 9924 2 hrs/2()00F. static FeA1+ Cr 29.0 93.6 11.6 12.9 3.5 4.4 .140 .014

1% HF, balance (4:5 by H... Weight).

* Slurry weight gain. 1.e.. gain in wt. of sample after dipplng lnslurry.

** Percent recovery-increase in sample weight divided by SWG and indicates the amount ofuluminum and chromium retained in sample.

TABLE 11 100 hrs. oxid. Percent Percent Percent Percent (wt. mg/cm Sample Sggce Percent Percent Cr Cr A1 A1 No. Conditions materials SWG recovery (top) (bottom) (top) (bottom) Top Bottom 9995 2 hrs/2000 F. static 2% FeAl+Cr 29.5 94.1 9.5 7.4 3.9 2.9 0.079 0.084

HF, balance Hz. (4:5 by v weight) 9996 2 hrsl2000 F. static 2% FeA1+ Cr 28.8 95.1 8.6 10.1 2.4 2.9 .074 .091

HF, balance Hz. (4:5 by

weight). 10180 2 hrs/2000 F. static 2.6% FeA1+ Cr 29.8 96.3 11.4 .171 .079

HF, balance H (4:5 by weight). 10181 2 hrs/2000 F. static 2.6% FeA1+ Cr 27.3 95.9 8.8- 9.7 .088 .083

HF, balance H (4:5 by

' weight). 10361 2 hrs/2000 F. static 0.5% FeA1+ Cr 22.6 94.3 6.000 .471

HF, balance H (4:5 by weight). 10250 2 hrs/2000 F. static 1% FeAl Cr 19.1 93.5 9.0 5.3 3.3 2.1 0.062 .142

HF, balance Hg. (4:5 by

- & l) 10471 2 hrs/2000 F. static 1% FeA1+Cr 28.8 95.3 9.5 6.8 .062 .079

HF. balance H (4:5 by weight).

TABLE 111 100 hrs. oxid. Sample Percent Percent Percent Percent (wt. mglcm No. Condition Source materials SWG recovery Cr (555511) 8576 4 hrsll900 F. flowing H 60 sec. FeA1+ Cr (1:3 by 24.1 35.6 3.7 6.0 0.189

pure HF at temp. weight). 8436 4 hrs/1900 F. flowing 2% HF 2 Fe A1+ Cr (1 :3 by 49.9 70.0 13.6 2.6 .065

' sec. pure HF at temp. werght 8425 4 hrsZ1900 F. flowing 2 0 HF 5 FeA1+Cr(1:2- by 40.0 51.5 11.2 4.0 .079

sec. pure HF at temp. weight). 8713 4 hrs/2000 F. flowing 2% HF 10 FeAl Cr (1:4 by 28.7 79.2 8.7 1.2 .097

' min. 53 sec. pure HF. 1 weight).

TABLE lV 100 hrs. oxid. Per- Percent Percent Percent Percent (wt. mglcm Sample Source cent Percent N Cr Cr Al Al No. Conditions materials SWG recovery (top) (bottom) (top) (bottom) Top .Bottom 9632 4 hrs/2000 F. static 5% FeAl+ Cr (4:5 28.9 92.3 10.4 8.4 4.0 4.3 0.190 0.115

' HF, balance H by weight). p I 9637 4 hrsl2000 F. static 1% FeAl Cr (4:5 30.6 93.8 13.9 7.2 4.2 4.2 .145 .082

HF, balance H laylyvgghl 96il 4 hrs/2000 F. static 1% F e r (3:5 28.1 H 87.7 9.1 8.3 .335 .350

HF, balance H by weight). 9672 2 hrsl2000 F. static 1% FeAl Cr (4:5 31.1 92.4 9.7 7.7 .083

HF, balance H by weight).

TABLE V 100 hrs. oxid. Per- Percent Percent Percent Percent (wt. mglcm Sample Source cent Percent Cr Cr Al Al No. .Conditions materials SWG recovery (top) (bottom) (top) (bottom) Top Bottom 10806 2 hrs/2000 F. static 1% CrAl (-70) 22.2 74.3 3.2 2.9 6.2 6.5 0.242 0.013

HF Bal H 10807 2 hrs/2000" F. static 1% CrAl (30-70) 16.0 79.3 .015 .014

Hfjililllz- 10808 2 hrs/2000 F. static 1% CrAl (30-70) 20.8 88.3 .012 .014

. HF+ Bal Hz. 10809 2 hrs/2000" F. static 1% CrAl (30-70) 22.7 84.7 5.5 4 1 4.9 9 1 .015 .020

HF+ Bal H 10893* 2 hrsl2000 F.'static 1% CrAl (30-70) 14.6 79.4 5 5 Y 10 10 .010 .033 HF Bal H Note: Resulting alloy may be found too brittle for some uses due to large amount of aluminum relative to the chromium.

TABLE V1 100hrs. oxid. Per- Percent Percent Percent Percent (wt. mg/cm) Sample Source cent Percent Cr Cr Al Al No. Conditions materials SWG recovery (top) (bottom) (top) (bottom) Top Bottom 10339 2 hrs/1900 F. static 1% FeAl FeCr 28.6 92.1

HF+ Bal1 l 4:7.l5 10363 2 hrs7-2000 F. static 0.5% FeA Ij- FeCr 26.8 90.8 0.81

HF+Ba1 H (417.15). 10376 2 hrs/2000 F. static 1% FeAl FeCr 29.6 89.1 .862 0.233

1 HF Bal H (4:7.15). 10387 2 hrs/2000 F. static 1% FeAl FeCr 30.5 81.9 .300 .232 HF Bal H (417.15). 10434 2 hrs/2000" F. static 1% FeAl FeCr 29.4 76.6

HF Bal H (417.15). 10439 3 hrs/2000 F. static 1% Fel+FeCr 28.5 88.1

HF Bal H2. (417.15) 1 a Note: FeCr (Low Carbon Ferrochrome 67.2% Cr) is preferred as a chromium source because it is less expensive than pure chromium.

Some Variables With reference to FIGS. 5,- 6 and 7, it can be seen that the variables of (1)time and temperature, (2) slurry composition and (3) atmosphere'composition are all inter-related and should be considered insofar as optimizing the subject invention for any particular assembly or material concerned.

FIG. 5 illustrates that the depth of diffusion may be obtained in shorter time intervals at higher temperatures with the converse also being true. FIG. 6 illustrates how diffusion appears to be best at a ratio of Fe-Al to Cr of about 4:5 as the relative'amount of chromium increases, up to about 4:5, then dropping off.

FIG. 7 indicates that a mix of about 1 percent HF with H is optimum in the case of source materials of Al-Fe Cr.

Materials FIG. 8 demonstrates a preferred composition best suited to the purpose of this invention as can be seen from the parts of the graph which fall into the central Slurrying Operations In the case of regenerator cores, different results may be obtained'depending on the slurry operation. Using sample cores of the type shown in FIGS. 1 and 2, wherein one face is a hot face, the other being a cold face because of the difference intemperature to which the faces areexposed in actual use, certain variables were found to provide certain results.

For example, controlled withdrawal of the core from the slurry during dipping was found to affect the l1 amount of slurry retained by the matrix and its distribution therein. Amount gained is shown in FIG. 10, a graph which demonstrates that the percent SWG, i.e.,

slurry weight gain resulting from dipping, was found to increase with withdrawal rate from the slurry.

Referring to FIGS. 11 and 12, the graphs thereof show that SWG increases with the viscosity of the slurry. Of course, other variables such as corrugation spacing, fold radius and passage uniformity in the matrix along with Al and Cr source particle size also affect SWG. A particle size of 325 mesh has been found to be acceptable in most cases, although this can vary a great deal.

Slurry distribution in the matrix of a core of the type shown in FIGS. 1 and 2 will not be completely uniform and will result in a compositional gradient from top to bottom across the matrix i.e., from one face to the other. This is acceptable and even desirable from an TABLE VII X-ray Fluorescent Analysis of Cr-Al Regenerator Cores SAMPLE Np. Cr %Al (Hot Face) Top 361-1 10.4 3.1 361-2 10.9 3.4 361-3 8.4 3.4 361-4 4.2 3611-5 8.7 3,1 361-6 9.7 4.2 361-7 7.1 3.0 (Cold Face) 361-8 7.8 42

Preferred Procedure for Processing a Full-Size Regenerator Core (17 inches in diameter and 3.5 inches thick) A slurry is prepared consisting of iron-aluminum and chromium powders in a 4:5 ratio (by weight) suspended in a vehicle of acrylic binder, hexane and toluene in a ratio of 6:521 with the addition of 0.25 percent aluminum palmitate. Viscosity is adjusted to about 230 (at 78 F.) centipoises, as measured with a Brookfield Viscometer.

A low carbon iron regenerator matrix assembly, preferably pre-bonded by a simple diffusion heat treatment as described above at decarburizing is coated with the above slurry by dipping involving a controlled withdrawal rate according to FIG. of about 6-8 inches/minute. A cleaning procedure'using low pressure compressed air, and blotting is used to remove the resultant drip edge. 'Drying with warm low'pressure air follows. A preferred slurry weight gain of about 25 to 30 percent should be obtained. Adjustment of viscosity may be used to influence retention. After cleaning up excess slurry, any component, such as hub, rim, etc. to be brazed to the core is assembled using a copper flake slurry. The slurried core is then placed in the diffusion container on an Inconel screen coated with stop-off and supported on an Inconel grid. Control samples are located at the periphery of the core andacover made (r316 stainless steel (0.015 inch) is welded on the container which may be Inconel also.

After testing for leaks, the container is purged with argon while being heated to about 700 to 800 F. for a minimum of 2 hours to remove the acrylic binder from the dried slurry. The cooled container is then evacuated to a few millimeters of mercury. The preferred diffusion atmosphere consisting of hydrogen and hydrogen fluoride (1 percent by volume balance substantially H is bled in until internal pressure is back to atmospheric, preferably slightly higher, then the retort is further purged by flowing the gas mixture throughit for an additional 1 5 minutes. The container gas outlet is then connected to an oil manometer to establish a static atmosphere and monitor pressure during the process.

Heating to about 2,000 F. (2,050 F. if copper brazing is to be done at the same time) then proceeds as an average heat-up rate of 400 to 500 F. per hour. Final temperature of about 2,000 F. is held for 2 hours. The static atmosphere is maintained by manipulation of the pressure regulator on the gas mixture cylinder so as to maintain a preferred height of about 4 to 6 inches in the oil manometer..After the holding period, the furnace is turned off and the container is cooled atthe highest practical rate. Upon reaching a temperature of approximately 1,000 F, the hydrogen-hydrogen fluoride atmosphere is purged from the container with an inert gas, preferably argon. After cooling to room temperature, the processed core is cleaned of loose residue by blowing with compressed air.

TABLE VIII Stagesof a Heating Cycle Time Heating Rate Final (Minutes) F.lmin. Temp. F. Notes 0-30 25-30 1000 Predominant action in this portion of 30-60 Rate changes 1400 cycle includes the from 25-30 down aluminum reactions, to about 6 formation and diffusion. 60-180 6 2050 Predominant action in this portion of cycle 180-240 0 2050 includes the aluminum (K) reactions, formation and diffusion. Cooling 420-510 7 I400 510 16-20 RT Having described the invention, the exclusive rights and privileges claimed are defined as follows:

1. The method of preparing oxidation resistant iron base alloy matrix structures allowed at least in portion thereof with Cr and Al, the method comprising the steps:

providing an iron-base alloy matrix assembly; applying a mixture of aluminum source material and chromium source material on said assembly, the aluminum source material being selected from the group consisting of Al-Fe alloys, Al-Cr alloys and mixtures thereof, the chromium source material being selected from the group consisting of Cr-Fe alloys, Al-Cr alloys, Cr and mixtures thereof, and heating the assembly in an atmosphere consisting essentially of H and HF at an elevated temperature for a time sufficient to cause the formation of Al and Cr fluoride salts and their reduction, and to also effect the diffusion of the Al and Cr, resulting from the reduction, into the iron-base alloy along with the bonding of the matrix assembly together to form an integrated structure. 2. The method according to claim 1 wherein the aluminum source is Al-Fe'alloy having a ratio of about 1:1.

3. The method according to claim 1 wherein the aluminum source is Al-Fe alloy and the chromium source is Cr, the relative amounts being such that there is at least as much Cr as Al-Fe.

4. The method according to claim 3 where the ratio of Al-Fe to Cr is about 4:5.

5. The method according to claim 1 wherein the atmosphere comprises about l-5 percent HF by volume, balance essentially H 6. The method of simultaneously diffusion alloying l0 and bonding regenerator assemblies, comprising the steps of:

providing a mixed atmosphere of H and HF for the assembly, and

heating the assembly in the atmosphere at an elevated temperature for a time sufficient to cause the formation of Al and Cr fluoride salts, their reduction to the respective elemental metals and to effect the diffusion thereof into the mild steel matrix along with the bondingtogether of the various parts of the assembly.

7. The method according to claim 6 wherein: the regenerator assembly is initially subject to a preliminary bonding step prior to the other steps, the initial step comprising holding the assembly together while heating it to about 1,600-l,700' F. to effecta-preliminary bonding of the assembly.

8. The method according to claim 6 wherein: the regenerator is of the flat cylindrical type used int certain turbine engines and a metal band is fastened around the circumference of the assembly followed by the heating thereof at an elevated temperature for a time sufficient to effect the preliminary bonding of the assembly together and thereafter performing the remaining stepsof the method.

9. The method according to claim 6 wherein the ma-.

trix is initially a low carbon mild steel.

10. The method according to claim 6 wherein the source of the chromium is an Cr-Fe alloy.

11. The method according to claim 6 wherein the source of aluminum is an Al-Fe alloy and the source of chromium is Cr and they are applied by means of a slurry containing the metals in powder form.

12'.- The methodaccording to claim 11 wherein the metals are-present in a ratio of from about 1:1 to a ratio wherein the relative amount of the Cr is larger than the relative amount of the Al-Fe.

13. The method according to claim 12 wherein the ratio of Al-Fe to Cr is about 4:5.

14. The method according to claim 11 wherein the slurry is applied by dipping the regenerator assembly into the slurry, withdrawing it and applying low pres sure compressed air to the assembly to clear the matrix passages, followed by a drying period.

15. The method according to claim 6 wherein the regenerator assembly is placed in a sealed container after the applying step and the container is purged with argon at an elevated temperature prior to further treatment.

16. The method according to claim 6 wherein the heating step is performed at a final holding temperature of about 2,000 F. in a static atmosphere.

17. The method according to claim' 16 wherein the H -HF atmosphere is purged with argon at about l,000 F. during cool-down after the final holding period and then cooling is allowed to proceed to. room temperature. 

2. The method according to claim 1 wherein the aluminum source is Al-Fe alloy having a ratio of about 1:1.
 3. The method according to claim 1 wherein the aluminum source is Al-Fe alloy and the chromium source is Cr, the relative amounts being such that there is at least as much Cr as Al-Fe.
 4. The method according to claim 3 where the ratio of Al-Fe to Cr is about 4:5.
 5. The method according to claim 1 wherein the atmosphere comprises about 1-5 percent HF by volume, balance essentially H2.
 6. The method of simultaneously diffusion alloying and bonding regenerator assemblies, comprising the steps of: providing a regenerator assembly having a matrix portion consisting, essentially of an iron-base alloy; applying a mixture of aluminum source material and chromium source material to the surface of said matrix portion, the aluminum source being selected from the group consisting of Al-Fe alloys, Al-Cr alloys and mixtures thereof, the chromium source being selected from the group consisting of Cr-Fe alloys, Al-Cr alloys, Cr and mixtures thereof, providing a mixed atmosphere of H2 and HF for the assembly, and heating the assembly in the atmosphere at an elevated temperature for a time sufficient to cause the formation of Al and Cr fluoride salts, their reduction to the respective elemental metals and to effect the diffusion thereof into the mild steel matrix along with the bonding together of the various parts of the assembly.
 7. The method according to claim 6 wherein: the regenerator assembly is initially subject to a preliminary bonding step prior to the other steps, the initial step comprising holding the assembly together while heating it to about 1,600*-1,700* F. to effect a preliminary bonding of the assembly.
 8. The method according to claim 6 wherein: the regenerator is of the flat cylindrical type used in certain turbine engines and a metal band is fastened around the circumference of the assembly followed by the heating thereof at an elevated temperature for a time sufficient to effect the preliminary bonding of the assembly together and thereafter performing the remaining steps of the method.
 9. The method according to claim 6 wherein the matrix is initially a low carbon mild steel.
 10. The method according to claim 6 wherein the source of the chromium is an Cr-Fe alloy.
 11. The method according to claim 6 wherein the source of aluminum is an Al-Fe alloy and the source of chromium is Cr and they are applied by means of a slurry containing the metals in powder form.
 12. The method according to claim 11 wherein the metals are present in a ratio of from about 1:1 to a ratio wherein the relative amount of the Cr is larger than the relative amount of the Al-Fe.
 13. The method according to claim 12 wherein the ratio of Al-Fe to Cr is about 4:5.
 14. The method according to claim 11 wherein the slurry is applied by dipping the regenerator assembly into the slurry, withdrawing it and applying low pressure compressed air to the assembly to clear the matrix passages, followed by a drying period.
 15. The method according to claim 6 wherein the regenerator assembly is placed in a sealed container after the applying step and the container is purged with argon at an elevated temperature prior to further treatment.
 16. The method according to claim 6 wherein the heating step is performed at a final holding temperature of about 2,000* F. in a static atmosphere.
 17. The method according to claim 16 wherein the H2-HF atmosphere is purged with argon at about 1,000* F. during cool-down after the final holding period and then cooling is allowed to proceed to room temperature. 